1. Field of the Invention
The present invention relates to a method of removing a native oxide film formed on a surface of a silicon wafer in a semiconductor manufacturing process, and more particularly, to a method of removing a native oxide film in a contact hole formed by applying an insulation film such as a boro-phospho silicate glass film (BPSG film) or a phosphorus doped glass film PSG film) on a silicon wafer according to a desired pattern. The method provides excellent etching selectivity with respect to the BPSG film or the PSG film.
2. Description of the Related Art
A semiconductor device manufacturing process might produce various contaminations, which adversely affect the operation characteristics of the device. Various attempts have been made for cleansing a silicon wafer surface at each step of the manufacturing process.
A native oxide film (SiO.sub.2) formed on a surface of a silicon wafer is one of the contaminations to the wafer surface. The native oxide film having a thickness of 10-20.ANG. is readily formed on the wafer surface left in the atmosphere. The native oxide film is also formed in the steps of cleansing, etching and so on of the semiconductor device manufacturing process.
A native oxide film left on a wafer surface on which electrodes, such as sources and drains, are to be formed prevents normal functioning of the electrodes. In addition, the native oxide film has to be completely removed from the silicon wafer surface in order that a metallic electrode having a low contact resistance can be formed.
The following two methods are known for removing from the wafer surface a native oxide film, which is formed in a contact hole of the wafer surface through application of a patterned insulation film such as a BPSG film or a PSG film on the silicon wafer. A first method is etching the native oxide film on the wafer surface according to a wet method by using buffered hydrogen fluoride (mixture of ammonium fluoride, hydrogen fluoride and water). According to the first method, the silicon wafer surface is etched by immersing the silicon wafer in buffered hydrogen fluoride solution stored in a bath of a wet station, i.e., like ordinary wet cleansing. In this first method, the etching selectivity of the native oxide film to the insulation film, including a BPSG film or a PSG film, is controlled through adjustment of the hydrogen fluoride ionic strength by adding an appropriate amount of ammonium fluoride to the buffered hydrogen fluoride. The silicon wafer should be rinsed with pure water after the etching process.
A conventional second method of removing the native oxide film in a contact hole on a silicon wafer is by vapor etching the native oxide film on the wafer surface using a mixed vapor of hydrogen fluoride and water (HF-H.sub.2 O). In the second method, the HF-H.sub.2 O mixed vapor is prepared by mixing vapor generated from separately prepared hydrogen fluoride and water, or by generating vapor from a previously prepared HF-H.sub.2 O mixed solution. The HF-H.sub.2 O mixed vapor is supplied into a chamber for etching in which the silicon wafer is housed, to etch the silicon wafer surface. The second method requires a rinsing process with pure water after the etching process because particles (reaction products) are liable to be produced in the etching process.
In the first method using the buffered hydrogen fluoride, however, particles in the chemical solution or pure water attach to the silicon wafer surface or inorganic/organic impurities in the chemical solution again attach to the wafer surface, causing deterioration of quality of the wafer. Because of the wet processing, the method is liable to cause a thicker native oxide film to form on the surface of the silicon wafer than a vapor processing. In this method, the etching selectivity of the native oxide film is controlled by preparing hydrogen fluoride ionic strength through the addition of ammonium fluoride to the buffered hydrogen fluoride. As a result, an etching rate of the insulation film, such as a BPSG film or a PSG film, is larger than that of the native oxide film. The etching rate of the native oxide film is set to be 10-15.ANG. per minute, for example, and that of the BPSG film or the PSG film is 300-800.ANG. per minute. Etching selectivity (an etching rate of a BPSG film or a PSG film/an etching rate of a native oxide film) of 20-80 is too large a value.
This method has another problem, that is, difficulty in introducing the chemical solution or pure water into a contact hole of sub-micron size having a diameter of 0.7-0.8 .mu.m. In particular, with a contact hole of about 0.5-0.6 .mu.m in diameter, the introduction is impossible unless a surfactant is used. Even if the chemical solution or pure water can be introduced into the contact hole, the introduction is not satisfactory. Therefore, the etching of the native oxide film in a contact hole on a silicon wafer according to the first method is slower than the ordinary etching processing. Furthermore, the first method requires a rinsing process as described above. Introduction of pure water into the contact hole is difficult and substitution of the liquid in the contact hole is accordingly unsatisfactory, which makes the desired rinsing processing impossible.
The second method, which uses HF-H.sub.2 O vapor, also has a problem in that the etching rate of the BPSG film or the PSG film, which are insulation films, becomes extremely large. Hydrogen fluoride has its ions dissociated in the presence of water to produce fluoride ions HF.sub.2.sup.-. The fluoride ions act on silicon dioxide SiO.sub.2, resulting in the etching of the silicon oxide film. The BPSG film or the PSG film, which are extremely hygroscopic, adsorbs much water if water exists in the atmosphere, as it does in the etching by HF-H.sub.2 O vapor. The action of the water in particular promotes etching of the BPSG film or the PSG film, resulting in a significant increase in the etching rate. With a native oxide film having an etching rate of about 10-12.ANG. per minute, for example, the etching rate of the BPSG film is not less than 1000-2000.ANG. per minute. The etching selectivity, as defined above, also becomes as large as 80-200.
An extremely large etching rate of the BPSG film or the PSG film causes the following problem. With reference to FIG. 1, consider the case where a native oxide film 122 is to be removed from the surface of a silicon wafer 120. The native oxide film 122 covers an inner bottom surface of a contact hole 126 formed by a patterned BPSG film (or a PSG film) 124 provided on the surface of the silicon wafer 120 as shown in FIG. 1.
There is no particular problem in etching the BPSG film 124 in the direction of its thickness because the thickness of the film is about 1 .mu.m, which is much larger than that of the native oxide film 122. However, if the sidewalls of the contact hole 126 formed by the BPSG film 124 are greatly etched (as indicated by dashed lines in FIG. 1), the contact hole 126 is enlarged by an etched area 128. Variation in size of the contact hole adversely affects the subsequent steps. In addition, an interface between the surface of the silicon wafer 120 and the BPSG film 124 could be damaged. These may cause significant defects in the quality of the wafer.
In the second method using HF-H.sub.2 O mixed vapor, water is left in the atmosphere. Because particles are liable to be generated in the etching step as described above, a rinsing process by pure water is required.